Important substances in the preparation of perfumes are fragrances of the indane and tetralin musk type such as 7-acetyl-1,1,3,4,4,6-hexamethyltetralin. These substances have excellent odour and fixative properties. They are stable to alkali and light, soluble in most solvents, substantially colourless and persistent and are also relatively cheap. They are commonly prepared by acetylation of the corresponding methyl-substituted indane or tetralin in a Friedel-Crafts reaction. In turn, many routes have been proposed for the preparation of these latter substances. Several have been proposed involving the formation of an alicyclic attachment to substituted cymenes which in turn may be obtained from 8-hydroxycymene.
It is known from U.S. Pat. No. 2,366,409 (Hercules) that 8-hydroxymenthene can be converted in the liquid phase to a mixture of 8-hydroxycymene and 8-hydroxymenthane by means of a disproportionation reaction. This reaction appears to be a combination of hydrogenation and dehydrogenation i.e. is a transfer hydrogenation. For every molecule of 8-hydroxymenthene which is dehydrogenated, enough hydrogen is produced to hydrogenate two molecules of 8-hydroxymenthene to 8-hydroxymenthane.
The Hercules reaction results in an approximate 2:1 mixture of respectively 8-hydroxymenthane and 8-hydroxycymene. We have confirmed the teaching in the Hercules patent that the pressure at which the reaction is carried out makes little difference to this ratio and pressures as low as 0.4 kPa at 73.degree. C. (reflux temperature) in the liquid phase will still give an approximate 2:1 ratio of products.